Rotary steerable drilling systems based on the “point-the-bit” principle, require a flexible joint in the steering unit to dynamically connect the lower part of the drill string (Bit Shaft end) to the main tool string. The bit shaft axis is tilted by a fixed or variable angle relative to the main tool axis, thereby “pointing” the drill bit in the desired direction of steering. The flexible joint allows the drilling fluid to be pumped through the center of the joint. A sufficiently large radial clearance is maintained between the tool outer diameter and the diameter of the well bore to allow the drilling fluid and the drill cuttings to flow back to surface. The flexible joint transmits the drilling loads (torque and axial load) from the main tool string to the drill bit.
In some rotary steerable drilling systems, for example, the universal joint uses a multitude of ceramic balls in pockets around the circumference of the bit shaft to transmit the drilling torque to a torque ring featuring corresponding cylindrical channels. Load is transferred through point/line contact between the ceramic balls and channels in the torque ring. The axial load is transmitted through a separate set of spherical thrust bearings away from the center of the universal joint. Two pairs of thrust bearings are required to allow compressive and tensional axial loads to be transmitted. Existing designs tend to be complex and limited in load capability, especially for smaller borehole sizes.